Light emitting diodes (LEDs) have begun to be used in traffic lights or tail lamps of vehicles. Consequently, the realization of a spatial wireless communication system through optical signals, in which LED arrays are used as light sources and, road information and information facilitating safe driving of the vehicles can be transmitted and received between the traffic light and the vehicles (road-to-vehicle) or between the vehicles (inter-vehicle or vehicle-to-vehicle), is greatly expected.
For example, as illustrated in FIG. 12(a), an optical-information acquisition element implemented by a photodiode (33, 31), which encompasses a p-type semiconductor layer 31 and an n-type surface-buried region 33 arranged on the semiconductor layer 31 is proposed (see Non-patent Literature (NPL) 1). A junction capacitor of the photodiode configured to generate signal charges is connected in parallel to the photodiode, and the junction capacitor serves as a charge-accumulation capacitor for accumulating the charges generated by photoelectric conversion. On the upper portion of the surface-buried region 33 (light-receiving cathode region), a p-type pinning layer 37 connected to ground potential (lower-level power supply) GND is arranged. Moreover, as shown on the right side of FIG. 12(a), on the surface of the semiconductor layer 31, an n-type charge-accumulation region 36, which serves as a floating diffusion region separated from the surface-buried region 33, is arranged, and an n-type reset-drain region 39 of a reset transistor is arranged, being separated from the charge-accumulation region 36. The charge-accumulation region 36 also serves as a reset-source region of the reset transistor. A first gate insulation film is formed on the semiconductor layer 31 between the charge-accumulation region 36 and the reset-drain region 39, and a second gate insulation film is formed on the semiconductor layer 31 between the surface-buried region 33 and the charge-accumulation region 36. On the first gate insulation film, a reset-gate electrode is arranged. Then, the charge-accumulation region 36, the reset-gate electrode and the reset-drain region 39 implement an nMOSFET, which serves as the reset transistor. On the second gate insulation film, a barrier-gate electrode is arranged, and with the semiconductor layer 31 as a source region, the barrier-gate electrode and the charge-accumulation region 36 serving as a drain region implement an nMOSFET, which serves as a barrier transistor.
FIG. 12(b) illustrates a potential profile of the conduction band at the surface portion of the semiconductor layer 31, when a voltage of a high level is applied to the barrier-gate electrode and consequently the barrier gate transistor is turned on and simultaneously, the voltage of the high level is applied to the reset-gate electrode and consequently the reset transistor is turned on. The carriers (electrons) generated in a charge-generation region (light-receiving anode region) are injected into the charge-accumulation region 36 that is lower in potential level than the surface-buried region 33. Since an impurity concentration of the surface-buried region 33 is set lower than an impurity concentration of the charge-accumulation region 36, the photodiode can be operated at a perfectly depleted potential, and the value of its capacitance is made independent of a response in the charge-accumulation region 36, and a parasitic capacitance CFD can be made small. For this reason, while sufficiently reserving the estate area of the photodiode, it is possible to respond to optical-communication signals at a high speed